Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression system are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage areas in commercial establishments. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodal. Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature at which the particular product being stowed in the cargo space needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved.
The basic components of a refrigerant vapor compression system include a refrigerant compression device, a refrigerant heat rejection heat exchanger, and a refrigerant heat absorption heat exchanger, and an expansion device, commonly an expansion valve, disposed upstream, with respect to refrigerant flow, of the refrigerant heat absorption heat exchanger and downstream of the refrigerant heat rejection heat exchanger. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in a conventional manner in accord with a refrigerant vapor compression cycle. Such refrigerant vapor compression systems may be designed for and operated in a subcritical pressure range or in a transcritical pressure range depending upon the particular refrigerant with which the system is charged.
In refrigerant vapor compression systems operating in a subcritical cycle, the refrigerant heat rejection heat exchanger functions as a refrigerant vapor condenser. However, in refrigerant vapor compression systems operating in a transcritical cycle, the refrigerant heat rejection heat exchanger functions as a refrigerant vapor cooler, commonly referred to as a gas cooler, rather than a condenser. Whether the refrigerant vapor compression system is operated in a subcritical cycle or in a transcritical cycle, the refrigerant heat absorption heat exchanger functions as a refrigerant evaporator. In operation in a subcritical cycle, both the condenser and the evaporator heat exchangers operate at refrigerant temperatures and pressures below the refrigerant's critical point. However, in refrigerant vapor compression systems operating in a transcritical cycle, the gas cooler operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while the evaporator operates at a refrigerant temperature and pressure in the subcritical range. Thus, for a refrigerant vapor compression system operating in a transcritical cycle, the difference between the refrigerant pressure within the gas cooler and refrigerant pressure within the evaporator is characteristically substantially greater than the difference between the refrigerant pressure within the condenser and the refrigerant pressure within the evaporator for a refrigerant vapor compression system operating in a subcritical cycle.
As refrigerant vapor compression systems are often operated in applications having a wide range of refrigeration load demand, it is known to provide a buffer volume into the system refrigerant circuit in which excess refrigerant collects and is stored during low load demand operation or during system standstill between periods of operation. In refrigeration vapor compression systems operating in a subcritical cycle, the buffer volume for storing refrigerant may be typically provided by incorporating a receiver into the refrigerant circuit to receive liquid refrigerant from the condenser or by incorporating an accumulator into the refrigerant circuit between the evaporator and the suction inlet to the compression device. In refrigeration vapor compression systems operating in a transcritical critical cycle, the buffer volume for storing refrigerant would not be provided by a receiver because the refrigerant heat rejection heat exchanger operates as a gas cooler, not as a condenser, thus the refrigerant leaving the refrigerant heat rejection heat exchanger is in a vapor state, not a liquid state.
U.S. Pat. No. 7,024,883 discloses incorporating an accumulator in the refrigerant circuit of a refrigerant vapor compression system operable in a transcritical cycle wherein carbon dioxide refrigerant is stored while the system is inactive. The accumulator is designed to have an optimal size for preventing over-pressurization of the system when the refrigerant is at a maximum refrigerant temperature and a maximum refrigerant pressure reached when the system is inactive.